在定义ListView的Selector时候，有个drawSelectorOnTop的属性，如果drawSelectorOnTop为true的话，Selector的效果是画在List Item的上面（Selector是盖住了ListView的文字或者图片），即Foreground前景。如果drawSelectorOnTop为false的话，Selector的效果是画在List Item的下面，即Background背景。由于项目中恰好需要自定义View，需要实现此效果。
本文借ListView的代码来剖析一下，
ListView完成此部分功能在frameworks\base\core\java\android\widget\AbsListView.java文件中。
用mSelector即ListView要画的Selector(资源文件)，而mSelectorRect则是想要画的区域。
/**
* Indicates whether the list selector should be drawn on top of the children or behind
*/
boolean mDrawSelectorOnTop = false; 决定画前景还是背景
/**
* The drawable used to draw the selector
*/
Drawable mSelector; ListView用中来显示Selector的Drawable，即ListSelector对应的XML文件
/**
* The current position of the selector in the list.
*/
int mSelectorPosition = INVALID_POSITION;
/**
* Defines the selector's location and dimension at drawing time
*/
Rect mSelectorRect = new Rect(); 用来画Selector的区域，即Selector画的位置AbsListView中构造方法中有获取selector
Drawable d = a.getDrawable(com.android.internal.R.styleable.AbsListView_listSelector);
if (d != null) {
setSelector(d);
}
//默认为false,画的是背景
mDrawSelectorOnTop = a.getBoolean(
com.android.internal.R.styleable.AbsListView_drawSelectorOnTop, false);/**
* Controls whether the selection highlight drawable should be drawn on top of the item or
* behind it.
*
* @param onTop If true, the selector will be drawn on the item it is highlighting. The default
* is false.
*
* @attr ref android.R.styleable#AbsListView_drawSelectorOnTop
*/
public void setDrawSelectorOnTop(boolean onTop) { //提供是否画前景或者背景的接口
mDrawSelectorOnTop = onTop;
}
/**
* Set a Drawable that should be used to highlight the currently selected item.
*
* @param resID A Drawable resource to use as the selection highlight.
*
* @attr ref android.R.styleable#AbsListView_listSelector
*/
public void setSelector(int resID) {
setSelector(getResources().getDrawable(resID)); 设置listSelector的XML文件
}
public void setSelector(Drawable sel) {
if (mSelector != null) {
mSelector.setCallback(null);
unscheduleDrawable(mSelector);
}
mSelector = sel;
Rect padding = new Rect();
sel.getPadding(padding);
mSelectionLeftPadding = padding.left;
mSelectionTopPadding = padding.top;
mSelectionRightPadding = padding.right;
mSelectionBottomPadding = padding.bottom;
sel.setCallback(this); //需要给Selector设置Callback
updateSelectorState();
}
/**
* Returns the selector {@link android.graphics.drawable.Drawable} that is used to draw the
* selection in the list.
*
* @return the drawable used to display the selector
*/
public Drawable getSelector() {
return mSelector;
}
void updateSelectorState() {
if (mSelector != null) {
if (shouldShowSelector()) {
mSelector.setState(getDrawableState());//更新Selector的状态
} else {
mSelector.setState(StateSet.NOTHING);
}
}
}这样就将Selector设置给ListView了，并且更新了drawable的状态。
接下来我们再看一下Android是如何将drawable画到ListView的Item上的。
在AbsListView中有个onTouchEvent的方法用来处理Touch事件，其中有一段代码就是确定Selector要画的区域。
if (mTouchMode == TOUCH_MODE_DOWN || mTouchMode == TOUCH_MODE_TAP) {
final Handler handler = getHandler();
if (handler != null) {
handler.removeCallbacks(mTouchMode == TOUCH_MODE_DOWN ?
mPendingCheckForTap : mPendingCheckForLongPress);
}
mLayoutMode = LAYOUT_NORMAL;
if (!mDataChanged && mAdapter.isEnabled(motionPosition)) {
mTouchMode = TOUCH_MODE_TAP;
setSelectedPositionInt(mMotionPosition);
layoutChildren();
child.setPressed(true);//设置List Item状态为 pressed
positionSelector(mMotionPosition, child);//确定画Selector的区域
setPressed(true); //设置ListView 的状态为pressed
if (mSelector != null) {
Drawable d = mSelector.getCurrent();
if (d != null && d instanceof TransitionDrawable) {
((TransitionDrawable) d).resetTransition();
}
}
if (mTouchModeReset != null) {
removeCallbacks(mTouchModeReset);
}
mTouchModeReset = new Runnable() {
@Override
public void run() {
mTouchMode = TOUCH_MODE_REST;
child.setPressed(false);
setPressed(false);
if (!mDataChanged) {
performClick.run();
}
}
};
postDelayed(mTouchModeReset,
ViewConfiguration.getPressedStateDuration());
} else {
mTouchMode = TOUCH_MODE_REST;
updateSelectorState();
}
return true;
} else if (!mDataChanged && mAdapter.isEnabled(motionPosition)) {
performClick.run();
}
}接下来看看positionSelector的实现，
void positionSelector(int position, View sel) {
if (position != INVALID_POSITION) {
mSelectorPosition = position;
}
//设置Selector的区域为List Item View的边界
final Rect selectorRect = mSelectorRect; selectorRect.set(sel.getLeft(), sel.getTop(), sel.getRight(), sel.getBottom());
if (sel instanceof SelectionBoundsAdjuster) {
((SelectionBoundsAdjuster)sel).adjustListItemSelectionBounds(selectorRect);
}
positionSelector(selectorRect.left, selectorRect.top, selectorRect.right,
selectorRect.bottom);
final boolean isChildViewEnabled = mIsChildViewEnabled;
if (sel.isEnabled() != isChildViewEnabled) {
mIsChildViewEnabled = !isChildViewEnabled;
if (getSelectedItemPosition() != INVALID_POSITION) {
refreshDrawableState();//根据View状态更新drawable的状态
}
}
}
private void positionSelector(int l, int t, int r, int b) {
mSelectorRect.set(l - mSelectionLeftPadding, t - mSelectionTopPadding, r
+ mSelectionRightPadding, b + mSelectionBottomPadding);
}好了现在已经决定了将selector画在哪里，Selector的状态也已经更新OK。
还差一步没有做，那就是到底是将其怎么画上面的呢？
答案就在AbsListView.java里的dispatchDraw方法里面。
@Override
protected void dispatchDraw(Canvas canvas) {
int saveCount = 0;
final boolean clipToPadding = (mGroupFlags & CLIP_TO_PADDING_MASK) == CLIP_TO_PADDING_MASK;
if (clipToPadding) {
saveCount = canvas.save();
final int scrollX = mScrollX;
final int scrollY = mScrollY;
canvas.clipRect(scrollX + mPaddingLeft, scrollY + mPaddingTop,
scrollX + mRight - mLeft - mPaddingRight,
scrollY + mBottom - mTop - mPaddingBottom);
mGroupFlags &= ~CLIP_TO_PADDING_MASK;
}
final boolean drawSelectorOnTop = mDrawSelectorOnTop;
if (!drawSelectorOnTop) { //将Selector画为背景
drawSelector(canvas);
}
super.dispatchDraw(canvas);// 用Canvas画ListView
if (drawSelectorOnTop) { //将Selector画为前景
drawSelector(canvas);
}
if (clipToPadding) {
canvas.restoreToCount(saveCount);
mGroupFlags |= CLIP_TO_PADDING_MASK;
}
}
private void drawSelector(Canvas canvas) {
if (!mSelectorRect.isEmpty()) {
final Drawable selector = mSelector;
selector.setBounds(mSelectorRect);//设置drawable画的区域
selector.draw(canvas); //使用canvas将drawable画上去
}
}看到这里，想必大家都已经明白如何画前景和背景了吧。在dispatchDraw之前调用就是画前景，在dispatchDraw之后调用就是画背景。
另外补充一下，本文并没有介绍动画部分，有兴趣的可以自己研究下。
总结一下，实现这个功能需要有三个步骤：
1.设置Selector，并更新状态（初始化时候）
2.确定Selector画的区域，设置View的状态，根据View状态，更新Selector的状态(一般是对Event的处理方法中)
3.使用Canvas在dispatchDraw中，将Selector画上去，画Drawable的时候需要先设置区域，再调用drawable的draw方法。
后面我再将View如何画背景和前景补上，今天就先到这里吧。
来源：oschina
链接：https://my.oschina.net/u/75011/blog/202291